I had the same injury many years ago…hyperextended my knee backwards while skiing and fractured the tibial plateau.
Not really an injury you want to mess with…if the fracture gets any worse, you can end up having to get surgery and putting screws in to hold the bone together. What sucks is that you can actually feel OK pretty quickly, but you still need to be non-weight bearing. It is a disconnect that is very frutrating for active people.
As for exercises, the short answer is to just suck it up. You are going to lose fitness, but you’ll bounce back quickly. Accept the loss of fitness vs. the risk of further injury. You may actually come back stronger after an extended break once you get rolling again. If you really need to do something, clear it with your doc first…but you may be able to so some swimming or core work as long as you aren’t loading your leg. Don’t do any one-legged stuff with your good leg because you will just casue a greater muscle imbalance once you get the all-clear to resume training.